1. Field of Invention
The present invention relates to a cancellation circuit that suppresses electromagnetic interference in a high speed circuit and, more particularly, to a cancellation circuit that cancels the magnetic field and couples with the electric field using a function generator.
2. Related Art
To increase the electromagnetic compatibility (EMC) of electronic products, one has to take account of the circuits for the power, signal lines, analogue signals, digital signals, and so forth when designing products. One crucial thing is the filter circuit, including the decoupling capacitor, the filter capacitor, the bypass capacitor, etc. Since the clock rate on the PC board is increasing (from 33 MHz to 133 MHz), normal filters for signals (including RC filters and LC filters) cannot achieve the object of canceling electromagnetic interference in a high speed circuit under the consideration of the possibility of affecting the signal quality.
In observation of the fact that the above-mentioned filters will seriously affect the signal quality of a high speed circuit, it is a feasible way to use differential signals to cancel electromagnetic interference in a high speed circuit. However, most current high speed circuit signals are single-ended signals, and their electromagnetic interference cannot cancel with that of the differential signals.
When a PCB is used in high speed digital circuit processing, it needs to process narrow pulses with rapid rising and falling times and avoid incorrect actions due to power damages. Therefore, a cancellation circuit for solving electromagnetic interference in a high speed circuit becomes very important.
Differential signals that are used in the cancellation circuit of the invention are generated from a clock pin of a function generator, which is phase shifted by a phase shifter and passes through a microstrip antenna or a stripline antenna so as to emit electromagnetic waves with inversed or shifted phases, canceling the originally existent electromagnetic waves.